1. Field of the Invention
This invention is in the field of lasers, and more specifically, the invention is related to methods and apparatus to combine multiple laser beams into a single high brightness beam.
2. Relevant Background
Directed energy weapons (DEW) based on delivery of laser energy to a target have been pursued for many years. Such laser beams must meet stringent criteria that include scalability to very high powers (e.g., powers of hundreds of kilowatts (kWs) to megawatts (MW) or more) in order to deliver sufficient cumulative energy on a target. DEW laser beams also require nearly diffraction limited beam quality to minimize the illuminated area at the target. Producing such beams from a single laser source is very difficult. The difficulties are due in part to the fluence (power or energy per unit area) often being high enough to destroy optics or optical components in the beam path. This is not surprising since a primary intent of DEW lasers is to produce destructive fluence (e.g., high power or energy per unit area). One clear objective for a designer of a DEW or other high energy laser is to generate high energy or destructive fluence at the target but not in the laser beam generator itself.
Another difficulty in creating beams of very high powers is that it is generally the case that beam quality degrades as laser power levels are increased. This degradation with increasing power is frequently a consequence of thermally induced distortions. Poor beam quality causes the laser area at the target to be larger than it would be for a perfect or non-distorted beam and consequently, in distorted or lower quality beams the fluence is reduced. As a result of these considerations, considerable effort has been directed to devising methods to combine the outputs from multiple lower power lasers into a single high power beam. In this description, the term “power” is typically used but may be thought of as meaning the more general phrase of “power or energy.” In creating a high power laser, the quantity that frequently matters more is often energy delivered to a target area although in DEW applications it is also frequently the case that the energy must be delivered in a certain amount of time to maximize lethality. High energy delivery can be achieved through highly energetic short duration pulses or through lower peak power pulses of long duration that in some cases last several seconds or more.
One significant consideration in combining beams to produce high power or energy is scalability, e.g., many separate sources often must be combined. A number of beam combination methods have been disclosed that rely on polarization combining two beams (see, for example, U.S. Pat. Nos. 4,982,166 and 5,172,264 to Morrow), but such combination methods are of little use for DEW applications which typically involve the combination of many beams.
Additionally, several scalable beam combination methods have been devised that can be divided into two broad categories: those relying on phased array concepts and those relying on spectral beam combination. Other non-scalable techniques do exist (e.g., temporal stacking of multiple pulses as described in U.S. Pat. No. 4,345,212 to Seppala and Haas), but these techniques require specific pulse formats and complex arrangements to implement. Phased array (PA) concepts rely on the creation of multiple laser beams whose phase can be controlled to a sufficiently high degree such that interferometric methods can then be utilized to combine the multiple beams into a single beam. Spectral beam combination techniques, on the other hand, use dispersive optical elements such as diffraction gratings to cause laser beams at different wavelengths to propagate in a single direction as one beam. Examples of spectral beam combination techniques have been disclosed in U.S. Pat. No. 6,697,192 to Fan et al. for example.
While some argue that spectral beam combination is generally superior, this technique has not proven useful or superior in many applications. For example, certain high-power applications, such as long-range coherent laser radar (ladar) applications, require the radiation to be single frequency, which precludes use of spectrally diverse methods. Another problem in implementing these techniques is that spectral beam combination requires an optical element to be inserted into the multiple beams, which then becomes susceptible to damage. As noted above, DEW applications are aimed at generating sufficient optical power to destroy objects. Hence, the placement of objects in the beam path is a great concern. One reason this issue is frequently not addressed by developers of spectral beam combination systems is that laboratory demonstrations are generally aimed at demonstrating physics principles rather than operational high power laser systems and are carried out at comparatively low power levels. Specifically, current combination demonstrations are generally performed at total power levels measured in Watts or at most hundreds of Watts, which is at least 3-5 orders of magnitude lower than what is required for operational DEW and other high power beam systems.
The same scalability and damage issues also apply to existing phased array or PA concepts. One subset of PA concepts uses diffractive elements, in particular phase gratings, to combine multiple beams. The general idea is that a phase grating can be constructed such that a single incident beam is split into multiple diffractive orders. By using this arrangement, reverse multiple beams interfering in a phase grating can be combined into a single beam. An example of such a method is disclosed in U.S. Pat. No. 4,933,649 to Swanson et al.
A second subset of PA concepts uses phase conjugation, e.g., conjugation based on stimulated Brillouin scattering or SBS, to phase lock multiple sources. Such concepts are disclosed, for example, in U.S. Pat. Nos. 6,385,228 to Dane and Hackel and 4,794,345 to Linford et al. However, as with spectral beam combiners and phase gratings, these approaches require insertion of optical elements into the beams, which is undesirable in DEW and other similar systems.
A third type of phased array is similar to phased arrays used in microwave radar as well as radio-telescopes. In these designs, multiple parallel beams are placed side by side to form a large area. Locking the phases of the individual beams to a common value ensures that the beam acts like a single beam with a larger area. A limitation of this approach is that of side lobes, which lead to energy deposition outside the intended target area. This represents an efficiency loss on the one hand and may also lead to collateral damage at unintended locations hit by the high power laser beam if this type of PA were used in a DEW system.
From the above discussion, it is clear that beam combination methods that do not require insertion of objects in the beam would be advantageous. The use of hollow waveguides to combine two beams has been demonstrated at low power by Jenkins and Devereux in U.S. Pat. No. 5,396,570, but the described method only discloses a method for combining Gaussian beams rather than a combination method for more general transverse intensity distributions. Gaussian profile beams are useful in many optical situations but are in other applications undesired because they have wide “tails” that prevent multiple beams from being positioned in proximity without interference. Truncation of laser beams produces intense, localized “hot spots” through diffraction, which are detrimental to safe scaling of the laser power to high levels. A further property of Gaussian beams is that they always remain Gaussian as they propagate through linear devices including mirrors, prisms, and lenses. More general beams, including super-Gaussian beams, do not behave in this manner, and consequently, a device that works with Gaussian beams often will not work in the same manner with a non-Gaussian beam. For example, the Fourier transform of a Gaussian beam is still Gaussian, whereas the Fourier transform of a higher order super-Gaussian beam is not super-Gaussian. Since imaging systems generally produce Fourier transformations (for example, the light distribution at the focal plane of a lens is the Fourier transform of the light distribution one focal length in front of the lens), it is not a priori a given that the appearance of a Gaussian beam profile is also an indication of a true imaging condition. Furthermore, a number of limitations arise from the waveguide geometry taught in the Jenkins patent that makes it difficult and cumbersome to apply for operational high power lasers. Another severe limitation of the Jenkins patent, as well as other existing waveguide configurations, is that they do not teach methods to carry out coherent combination of beams that are not a priori mutually coherent. Simply inputting multiple beams into a combiner without adequate phase control is not sufficient to ensure the emergence of a single coherent beam. Without phase control, the output will on average simply be the sum of intensities as if no coherent summation occurred. This is unacceptable in the construction of high power lasers of DEW and similar applications.
Waveguides are increasingly used in very low power telecommunications systems. In this area, several devices have been disclosed that perform certain beam splitting or beam combination functions but do not enable operation of a high power laser at high efficiency. Such devices include a hexagonal geometry device described in U.S. Pat. No. 6,125,228 which uses “kaleidoscope” effects to produce multiple beams and is aimed at wavelength division multiplexing (WDM) and similar low power applications. A further example of an integrated optical device for WDM applications is disclosed by Tayag and Batchman in U.S. Pat. No. 5,862,288. Another tapered waveguide device is described by Bouda in U.S. Patent Application 2002/0114572 A1. This device uses non-adiabatically tapered waveguides to produce beam splitting functions with an integrated optical device. The non-adiabatic waveguide is essential to the described device in order to produce a uniform illumination of multiple subsequent waveguides, but such a waveguide may be highly detrimental to the beam combination devices developed for DEW or other high power beam systems. For example, Bouda illustrates coupling from one input beam to a multiplicity of output beams where the coupling loss is measured in several decibels (dB). One dB equals 21% loss and 2 dB equals approximately 37% loss. Such high losses may be acceptable for low power applications but are generally unacceptable for high power lasers. One reason is that electrical power to drive the laser is very limited and must be used efficiently. Another reason such losses are unacceptable relates to thermal management. A high power laser operating at 500 kW that loses 20% of the light before transmission would need to safely dispose of 100,000 W of laser power, which is a very significant power level.
Self-imaging in an optical tunnel is described by Bryngdahl in U.S. Pat. No. 3,832,029, but Bryngdahl does not describe requirements to use such devices for coherent beam combination. Similarly, U.S. Pat. No. 4,087,159 to Ulrich describes a number of self-imaging waveguide devices, but it does not teach a system that can be scaled to high power and that can coherently combine many beams into one higher power beam.